1. Technical Field
This invention is in the field of magnetic sensors which detect a magnetic field or magnetic field change and find use in linear or rotary motion detection, electrical current sensing, linear or rotary position sensing, magnetic imaging, magnetic recording read/write heads, magnetic recording media as well as general magnetic field sensing.
2. Background of the Invention
Because of the many applications of magnetic field sensors, there is a long history of technical development of materials and means to measure fields of various strengths. No one sensor can perform every function well. Factors such as size, weight, power consumption, and cost should be typically minimized by a field sensor. Sensitivity, linearity, bias, stability, reliability, and operating temperature and frequency range are factors that should be typically optimized. As with any instrument, it is usually difficult to achieve all of these characteristics in one device.
The most common magnetic sensors used in a variety of applications are the Hall effect sensor and the variable reluctance coil. The drawback of variable reluctance devices is that they generate signals proportional in size to the time rate of change of magnetic flux. The signal size therefore decreases with decreasing speed, and below a certain flux change rate, the signal disappears into the noise. Hall effect devices generate a very small raw signal because of low field sensitivities (0.5xcx9c5 mV/100 Oe applied field), and the device performance is strongly temperature dependent. These features mandate signal conditioning, and require that a certain minimum field be available for device operation.
The concept of combining the magnetostrictive materials and piezoelectric layers for highly sensitive magnetometer was first introduced by Mars D. Mermelstein in 1986. In his patents U.S. Pat. No. 4,769,599 and U.S. Pat. No. 5,130,654 a magnetometer was disclosed as a device using piezoelectric resonator to create a standing stress wave in the sensing magnetostrictive ribbon and using a pickup coil to read-out the electromotive force. A minimum detectable field gradient of 7.7 pT/cm Hz was achieved in this device by using a differential amplifier technique.
In U.S. Pat. No. 5,675,252, a device called piezomagnetometer was disclosed. In that device a stack of 201 alternating piezoelectric and magnetostrictive layers was used, in which 100 pairs of piezoelectric-magnetostrictive layer capacitors are connected electrically in parallel to increase the charge storage by raising the effective capacitor plate area. The device requires multiple layers that are placed in a permanent biasing magnetic field normal to the layer surface. The resolution as high as 1 pT/cm Hz was reported.
The present invention concerns passive solid-state magnetic sensors based on a combination of magnetostrictive material and piezoelectric material. The sensors can be mass produced at low cost in comparison with any existing magnetic sensor technologies including variable reluctance coils, Hall-effect devices, magnetoresistance semiconductors, and the most recently developed giant magnetoresistance (GMR) metal multilayers. Such a magnetostrictive/piezoelectric sensor need not consume any electrical power and with a field sensitivities larger than 10 mV/Oe being obtainable. Applications including digital speed sensor, digital flow sensor, and electrical current sensor have been demonstrated.
According to one aspect, the present invention concerns a device with (A) laminated structure, (B) planar structure, and (C) fiber composite structure. The laminated structure device is made by one piezoelectric layer sandwiched with two magnetostrictive layers or one magnetostrictive layer sandwiched with two piezoelectric layers.
The three-layer device is preferably connected electrically in series to increase the voltage by raising capacitor plate separation. The planar structure device is made by patterned piezoelectric strips on a magnetostrictive substrate. The fiber composite structure device is made by piezoelectric fibers surrounded by magnetostrictive materials. These sensors are designed for general magnetic field detection purposes in applications ranging from speed, flow, and electrical current detection to the information storage and imaging. The advantages of the inventive sensors over the competitive technologies are passive solid-state, high field sensitivity, wide dynamic range up to several thousands of Oersted, and low-cost in manufacturing.
Generally, the present invention uses a piece of piezoelectric material in contact with a magnetostrictive material. The magnetostrictive component strains in response to a magnetic field. This strain couples to the piezoelectric element causing it to produce an electrical output signal.
One important characteristic of embodiments of the invention is that the sensitivity of the device and the operating magnetic field range can be adjusted through material properties and structure designs for a variety of applications. The important variables used in the design include 1) selecting magnetostrictive materials with appropriate properties, 2) selecting piezoelectric materials with optimal properties, 3) determining the optimal number of capacitive elements, 4) selecting the appropriate size of both magnetostrictive and piezoelectric elements, 5) designing the geometry of the structure, and 6) establishing the most efficient bonding and packaging methods. The inventive magnetic sensors can be widely used in replacement of Hall-effect sensors, variable reluctance coils, and magnetoresistive devices. The particular applications of the inventive magnetic sensors are (a) speed detection and controls for rotary machines including automobiles, airplanes, locomotives, etc., (b) flow meters for reading and controls of liquid or gas flows, (c) electrical current meters for reading and controls of electrical power usage, and (d) micromagnetic field sensors for magnetic recording/reading heads, magnetic recording media and magnetic imaging devices.
The above and other features of the invention including various novel details of construction and combinations of parts, and other advantages, will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular method and device embodying the invention are shown by way of illustration and not as a limitation of the invention. The principles and features of this invention may be employed in various and numerous embodiments without departing from the scope of the invention.